Process for carburization of iron and steel



" Patented Jan. 29, 1952 UNITED STATES "PATsNTP FFIcE,

a PROCESS FOR'CARBURIZAT'ION OF IRON AND STEEL l Axel G. E. Hultgren, Djursholm, Sweden I No Drawing. Application May 6, 1949, Serial No. 91,872. InSweden November 2, 1948 e 2 Claims. (01. 148 19) For pack carburizing, iron and steel objects are commonly embedded in special carburizing compounds consisting of powdered or' crushed charcoal and/or coke which has been intimately mixed with an activator or accelerator, for instance barium carbonate, and annealed in such a manner that the carbonaceous substance and the activator are not separated from each other.

During the continued investigation of this problem (of the mechanism of carburizing) it has been ascertained that it is possible to carry out a great number of carburizing operations in succession each time replenishing the carburizing substance with a certain amount of carbonaceous substance to which no activator or en ergizer has been added, provided in the first operation in the series a sufficient amount of activator was employed mixed with the carbonaceous substance or, an activator with spreading power was used, applied in one or more local places in the carburizing vessel. In other words, a carburizing mixture used once can be used again with advantage after mixing with an appropriate quantity of fresh carbonaceous substance (charcoal of coke). This method or admixture may be used in a great number of successive carburizing anneals without any noticeable decrease in depth of carburizati'on.

The quantity of fresh carbonaceous substance added each time should not be less than a certain minimum which varies with the nature of the substance (probably with its effective surface area per unit volume). For charcoal the minimum volume is of the order of per cent of the volume of the used mixture, for coke somewhat more.

The procedure according to the invention is as follows:

When starting, a known carburizing agent containing an energizer or charcoal, coke or the like with locally added activator having a spreading power is utilized for a first carburization. After this first carburization only charcoal or coke or a mixture of both is added and a new carburization of other objects is performed after which fresh charcoal, coke or the like is added again to the residual carbonaceous substance and so forth.

If after a great number of carburizing anneals the depth of carburization in the objects treated should tend to decrease, the carburizing power of the mixture may be restored by adding a moderate amount of activator or of a carburizing compound of the well-known type.

This discovery upon which the present invention is based is at variance with the underlying thoughts reflected in the present carburizing practice. Regarding this practice, the following statements may be quoted (Pack Carburizing by H. W. McQuaid, Metals'Handbook, 1948' edi- 'tion, American Society for Metals, Cleveland, 0.,

pp. 686-687). l

Because of the loss of the carbonate energizer the powdering of the compound and the burning in service, new compound must be added before the carburizer is used again. a

utilize at least five parts of old compound with one part of newand some plants. are operating with even smaller additions, of new compound. In many plants, however, the conditions are such that more new compound is required and it is not unusual for plants to make an addition of one partof new compound to'three part of used compound. a

Since the amount of new compound to be used is determined by theamount of energizer retained in the used compound, the methodof incorporating the energizer is important."

- The statements quoted may now be scrutinized inthe light ofthe investigation 'that' led up to the present invention: a t

It is true that a carburizing compound used once for carburization has lost some of its carburizing power. It is also true that that power will be regained by admixing a certain minimum amount of fresh compound. But the function of the fresh compound added is not to introduce new energizer to replace a similar amount lost during carburization, as is proven by the fact that the addition of an appropriate amount of fresh charcoal or coke with no energizer added, is just as efiicient in restoring the carburizing power, not only once but repeatedly for many carburizing anneals in succession. The consumption of energizer or activator during carburizing is, therefore, extremely small provided the box is properly sealed and such amount as may be needed to compensate for the consumption can be added at infrequent intervals.

Hence it may be concluded; (1) that the role of the energizer during carburizing is that of a catalyst, the amount of which remains undiminished if it is not Wasted away; and (2) that the waste, which is small, is due to dusting in handling and/or to evaporation and leakage through; the seal.

The fact that a certain minimum amount of fresh charcoal or coke-about 10 per cent of the mixture by weight for charcoal-must be added for the carburizing power to be completely restored is probably connected with the extent of the e fiectivesurfacearea of the carbonaceoussubstjance in the mixture. During the carburizing anneal this area decreases and the substance becomes less bulky as evidenced by the increase in density. In one case examined the density of a charcoal-potassium carbonate mix ture (60 40 by weight) was originally 0.3 g./cm. and increase during 40 carburizing tests'using additions of fresh charcoal, to 0.7 g./cm. Fresh charcoal or coke has considerably greater effective surface area than the used material and'thus a moderate addition will bring the total area of the mixture up to the minimum required for efficient carburizing. The -function of that carbonaceous surface is, of course, to turn the CO2 formed in the carburizing reaction at the steel surface into CO as fast as it is formed and thus to keep the level of the CO content in the gas as near as possible to the equilibrium value in contact with the carbonaceous substance.

If, as is common practice when using an ef ficient carburizing compound, the work pieces are packed fairly close, the box is well sealed and is not uncovered while hot the carbon consumption will go mainly to supplying the large 'steeljsurface with its proper amount and only a energizer; said solid carbonaceous material be ing selected from the class consisting of char coal and coke, utilizing said mixture. in at least terial from said class substantially free from added energizer in sufiicient quantity to provide the additional surface area per unit volume of the resulting mixture to restore at carburizing temperature the rate of regeneration of carbon dioxide to carbon monoxide required to substantially restore the carburizing properties of the agent, using the resultant mixture in one or more additional carburizing operations, and repeating the cycle of adding carbonaceous material from said class substantially free from added. energizer to used mixtures and reusing the thus restored agent a plurality of times in the, absence of additional energizer.

2. The method of effecting a series of carburizing operations which includes initially providing a carburizing agent comprising a mixture of solid carbonaceous material and an added energizer, said solid carbonaceous material being selected from the class consisting of charcoal and coke, utilizing said mixture in at least one carburizing operation, admixing with the used mixture at least about ten percent of additional solid carbonaceous material. from said class substantially free from added ener- REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date r 949,441 Rodman Feb. 15, 1910 1,310,918 Mebane July 22 1919 1,312,339 Mebane Aug. 5, 1919' 1,317,728

OTHER REFERENCES Steel Treating Practice, by Sherry, Mc- Graw-Hill Book 00., N. Y., 1929 pages 190-191, 198.

Rodman Oct. 7,1919 

1. THE METHOD OF EFFECTING A SERIES OF CARBURIZING OPERATIONS WHICH INCLUDES INITIALLY PROVIDING A CARBURIZING AGENT COMPRISING A MIXTURE OF SOLID CARBONACEOUS MATERIAL AND AN ADDED ENERGIZER, SAID SOLID CARBONACEOUS MATERIAL BEING SELECTED FROM THE CLASS CONSISTING OF CHARCOAL AND COKE, UTILIZING SAID MIXTURE IN AT LEAST ONE CARBURIZING OPERATION, ADMIXING WITH THE USED MIXTURE ADDITIONAL SOLID CARBONACEOUS MATERIAL FROM SAID CLASS SUBSTANTIALLY FREE FROM ADDED ENERGIZER IN SUFFICIENT QUANTITY TO PROVIDE THE ADDITIONAL SURFACE AREA PER UNIT VOLUME OF THE RESULTING MIXTURE TO RESTORE AT CARBURIZING TEMPERATURE THE RATE OF REGENERATION OF CARBON DIOXIDE TO CARBON MONOXIDE REQUIRED TO SUBSTANTIALLY RESTORE THE CARBURIZING PROPERTIES OF THE AGENT, USING THE RESULTANT MIXTURE IN ONE OR MORE ADDITIONAL CARBURIZING OPERATIONS, AND REPEATING THE CYCLE OF ADDING CARBONACEOUS MATERIAL FROM SAID CLASS SUBSTANTIALLY FREE FROM ADDED ENERGIZER TO USED MIXTURES AND REUSING THE THUS RESTORED AGENT A PLURALITY OF TIMES IN THE ABSENCE OF ADDITIONAL ENERGIZER. 